The use of peroxygen bleaching agents for washing clothes and other household articles has long been known. They are particularly valuable for removing stains having a significant content of colouring matter, for instance, tea, coffee, fruit, wine and cosmetic stains. Commonly, the bleaching agent takes the form of a peroxy salt such as sodium perborate or sodium percarbonate. This is typically added to a laundry detergent composition at a level in the range from about 5% to about 35% weight.
The effectiveness of peroxygen bleaching agents is known to be very variable, however, and is greatly affected by the level of heavy metal impurities in the wash water. Indeed, in the absence of these impurities, peroxygen bleaching agents have essentially minimal bleaching activity. Large quantities of heavy metal impurities, on the other hand, promote extensive decomposition of the bleaching agent with release of gaseous oxygen. For this reason, it has been common to add a sequestering agent such as ethylenediaminetetraacetic acid (EDTA) or its salts to provide a more uniform level of free heavy metal ions in solution. The effect of these sequesterants under normal conditions, however, is not only to control bleach decomposition but also to suppress the rate and level of bleaching activity.
A number of attempts have been made in the art to boost bleach performance by deliberate addition of heavy metal materials during the manufacturing process. Thus, in GB-A-984459 a combination of a copper salt and a sequestering agent having a copper dissociation constant in the range from -11 to -15, is used together with a water-soluble perborate bleaching agent. The dissociation constant of the complex is such as to provide a level of free copper ions in solution in the range necessary for activation of the perborate. Unfortunately, however, the buffering capacity of the sequestrant in this type of system is relatively weak with the result that significant variation in the level of free copper ions can still occur. Where, on the other hand, a sequestrant of greater chelating powder is used, such as EDTA, the level of free heavy metal ions in solution is reduced to such an extent that activation of the bleaching agent is minimal; in other words, the bleaching agent is "overstabilised".
In another approach described in GB-A-1,565,807, certain preformed iron (III)/chelate complexes are described for use with hydrogen peroxide bleach liberating persalts and are said to have a pronounced activating effect on the peroxygen bleach. The materials specified are iron (III) complexes of ethylenediaminetetraacetic acid, nitrilotriacetic acid, diethylenetriaminepentaacetic acid, and hydroxyethylethylenediaminetriacetic acid. This approach also suffers drawbacks however. In particular, the iron/chelate complexes are found to produce a significant increase in the level of fabric damage as a result of localised bleach catalysis at the fabric surface. Moreover, although bleach enhancement can be observed under ideal conditions (nil water hardness, "clean" wash loads), the chelate system is unable to handle the significant variations of heavy metal content introduced in the wash load or wash solution --in other words the system lacks robustness. Other deficiencies of the chelate system include inadequate fabric whiteness end-result, essentially nil bleach enhancement in lower temperature wash cycles (less than 60.degree. C.), and incompatibility with organic bleach activator materials commonly used for boosting low temperature wash performance.
It has now been discovered that the fundamental cause of these various performance deficiencies is one of complex instability. Thus under the pH and oxidising conditions typical of a laundry detergent or bleaching composition, the complex degrades both by hydrolysis and oxidation with formation and precipitation of ferric hydroxide. Moreover, Applicants have established that by selecting certain iron/chelate complexes having high hydrolytic and oxidative stability, it is possible to secure bleach catalytic enhancement without the adverse side effects displayed in the art.
The present invention therefore provides a bleaching auxiliary for use with a peroxygen bleaching agent or laundry detergent, the auxiliary being environmentally-acceptable and providing improved control of bleach activity over the range of wash temperatures, water hardness and soil load, with significant reduction in fabric damage and with improved fabric whiteness end-result. It also provides laundry bleaching and detergent compositions having more effective and efficient usage of peroxygen bleaching agent, thereby delivering an increased bleaching performance for any given level of peroxygen bleach, or minimising the level of peroxygen bleach required for any given level of bleaching end-result performance. The invention also provides a bleach auxiliary system for catalysing bleach activity which is fully compatible with organic peroxyacid bleach precursors.